A key functional aspect of the motor neuron projection pattern out of the CNS is that each neuron’s axon innervates only one muscle, but it branches to innervate multiple fibers within that muscle. Moreover, each extrafusal muscle fiber in mammals is supplied by only one α motor neuron. Thus a motor unit can be defined as an α motor neuron and all of the skeletal muscle fibers that its axon supplies. The motor unit can be regarded as the basic unit of movement because when an α motor neuron discharges under normal circumstances, all of the muscle fibers of the motor unit contract. That is, the safety factor (see ) of the neuromuscular junction is greater than 1, and so each action potential in the motor neuron axon triggers an action potential in every muscle fiber of the motor unit.
Of importance is that the average size of the motor unit (i.e., the number of muscle fibers innervated by an axon) varies between muscles, depending on how fine a control of the muscle is required. For finely controlled muscles, such as the eye muscles, an α motor neuron may supply only a few muscle fibers. However, in a proximal limb muscle, such as the quadriceps femoris, a single α motor neuron may innervate thousands of muscle fibers.
The muscle fibers that belong to a given motor unit are called a muscle unit. All the muscle fibers in a muscle unit are of the same histochemical type (i.e., they are all either slow-twitch [type I] or fast-twitch [type IIA or IIB] fibers). For an in-depth description of muscle fiber types, see . Of importance in this chapter is that a number of physiological properties are correlated with this histochemical classification scheme. In particular, slow-twitch fibers, which contract and relax slowly as implied by their name, also generate low-force levels but essentially never fatigue. In contrast, the fast-twitch fiber contract and relax rapidly, generate higher levels of force, and fatigue at varying rates.
The first motor units to be activated in many cases, either by voluntary effort or during reflex action or just
CHAPTER 9 Organization of Motor Function to maintain posture, are those with the smallest motor axons. These motor units contain slow-twitch fibers and thus generate the smallest contractile force, allowing the initial contraction to be finely graded. These units tend to be active much of the time, if not continuously, and so their lack of fatigability makes good functional sense.
As more motor units are recruited for a motor act, motor neurons with progressively larger axons become involved, and these axons synapse onto the fast-twitch fibers. Thus they generate progressively larger amounts of tension. The most powerful such motor units are typically recruited only for tasks requiring large amounts of force (e.g., sprinting, jumping, and lifting a heavy weight), tasks that people can perform for only for short periods of time.
The orderly recruitment of motor units helps the CNS generate a large range of forces and also maintain relatively precise control at the different force levels. This recruitment pattern is called the size principle because the motor units are recruited in order of motor neuron axon size. The size principle depends on the fact that small motor neurons are activated more easily than are large motor neurons. Recall that if an excitatory synapse is active, it opens channels in the postsynaptic membrane and causes an excitatory postsynaptic current. The same size excitatory postsynaptic current generates a larger potential change at the initial segment of an axon of a small motor neuron than it does at a larger motor neuron, simply as a consequence of Ohm’s law (V = IR) and the fact that smaller motor neurons have higher membrane resistance than larger motor neurons do. Thus because excitatory postsynaptic potentials in the CNS are small and need to summate to reach threshold for triggering spikes, as the level of synaptic bombardment rises from zero, the resulting depolarization will reach spiking threshold in smaller motor neurons first. As the size principle is usually obeyed, this assumption generally appears to hold; however, there can be exceptions, and in these cases, the descending motor pathways presumably must provide differing levels of synaptic drive to the different-sized motor neurons.